1. Field of the Invention
The present invention relates to photoelectric conversion devices and electronic devices having the photoelectric conversion devices. In particular, the present invention relates to photoelectric conversion devices in which output signals are logarithmically converted and electronic devices having the photoelectric conversion devices.
2. Description of the Related Art
A large number of photoelectric conversion devices generally used for detecting electromagnetic waves are known. For example, photoelectric conversion devices having sensitivity to ultraviolet rays to infrared rays are generally referred to as optical sensors. Among optical sensors, an optical sensor having sensitivity to a visible light region with a wavelength greater than or equal to 400 nm and less than or equal to 700 nm is particularly referred to as a visible light sensor. A large number of visible light sensors are used for devices which need illuminance adjustment, on/off control, or the like depending on human living environment.
In some display devices, ambient brightness of the display devices is detected so that display luminance is adjusted. This is because with optical sensors, visibility can be improved or wasted power consumption can be reduced by detecting ambient brightness and obtaining appropriate display luminance. For example, examples of display devices having optical sensors for adjusting luminance are mobile phones, computers provided with display portions, and the like. In addition, not only ambient brightness of the display portions but also the luminance of display devices, in particular, the luminance of backlights of liquid crystal display devices is detected with optical sensors so that the luminance of display screens is adjusted.
An optical sensor, which is a photoelectric conversion device, uses a photoelectric conversion element such as a photodiode for a light sensing portion, supplies photocurrent generated in the photoelectric conversion element to a resistor, and can detect light intensity in accordance with obtained output voltage (see Reference 1). Further, the photocurrent generated in the photoelectric conversion element is amplified in an amplifier circuit so that weak light is detected. A current mirror circuit is used as the amplifier circuit, for example (see Reference 2).
In a photoelectric conversion device disclosed in Reference 2, when light intensity of weak light to strong light is to be detected, the range of photocurrent amplified is wider. Therefore, in the case where photocurrent amplified by external load resistance or the like is converted into voltage, output voltage increases linearly with respect to illuminance. Accordingly, when output voltage is to be obtained in a wide illuminance range, several milli-volts are obtained in the case of weak light, and several volts are obtained in the case of strong light. Thus, due to limitations on a circuit (e.g., power supply voltage), it is difficult to broaden the dynamic range of illuminance as an optical sensor.
Meanwhile, in order to broaden the dynamic range of illuminance in a photoelectric conversion device, output having the level of voltage subjected to logarithmic compression (hereinafter referred to as output voltage) is obtained by supplying photocurrent generated in a photoelectric conversion element to a diode element (see Reference 3). Note that logarithmic compression refers to a technique by which the amount of output current or the level of output voltage is obtained as a logarithmic function with the illuminance of light which enters a photoelectric conversion element, i.e., the amount of photocurrent used as a variable.